My opinion? Leave them be. The engine will run just fine just fine the way they are and any tiny benefit of clean piston tops is not worth the risk of disturbed grit getting into places in the engine where it shouldn't be.

One piston with excessive carbon suggests a rich mixture in that cylinder. That in turn suggests an injector with a bad spray pattern or a worn pintle...either way, too much fuel in that cylinder. When the O2 sensor detects a rich mixture it causes the ECU to lean out the overall air/fuel ratio, so that the other cylinders run a bit lean. This may be why you have "white" deposits on those pistons.
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Bob: Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), 98 S90 (recently sold) and 2010 XC60. Also '77 MGB and four old motorcycles